Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and (in the case of Si) lower frequency applications. These semiconductor materials may be less well suited for higher power and/or high frequency applications, however, because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.
For high power, high temperature and/or high frequency applications and devices, wide bandgap semiconductor materials such as silicon carbide (2.996 eV bandgap for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV bandgap for GaN at room temperature) are often used. These materials, typically, have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.
A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which is also known as a modulation doped field effect transistor (MODFET). These devices may offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped (unintentionally doped), smaller bandgap material and can contain a very high sheet electron concentration in excess of, for example, 1013 carriers/cm2. Additionally, electrons that originate in the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering. This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications.
High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to polarization in the AlGaN. HEMTs in the GaN/AlGaN system have already been demonstrated. U.S. Pat. Nos. 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures and methods of manufacture. U.S. Pat. No. 6,316,793, which is commonly assigned and is incorporated herein by reference, describes a HEMT device having a semi-insulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure.
The passivation layer in HEMT devices has traditionally been formed on the top surface of the barrier layer in an ex-situ manner, meaning in a different reactor chamber after removal of the device from the epitaxial growth reactor. This may result in small pit formation in the barrier layer at points where screw and edge dislocations traverse the interface between the channel and barrier layers. The diameter of these pit openings are small (e.g., on the order of twice the thickness of the barrier layer), but clearly discernable on a 2×2 micron-square AFM scan.
Another step in the fabrication of nitride-based transistors is the formation of ohmic contacts for such transistors. The formation of ohmic contacts has, typically, required high annealing temperatures (e.g. 750° C.). Such high annealing temperatures may damage the materials and/or the device.
For example, in conventional devices utilizing high annealing temperatures when forming ohmic contacts, the sheet resistance of a gate region (defined as the active device region between the two contacts) of AlGaN and/or GaN layers typically increases in comparison to sheet resistances of the AlGaN and/or GaN layers as-grown. Such an increase in sheet resistance is believed to detrimentally affect the device.
U.S. Pat. No. 6,498,111 discloses a method for protecting the surface of a semiconductor material from damage and dopant passivation in which a barrier layer of dense material is deposited on the semiconductor material shortly after growth in a growth reactor such as a MOCVD reactor, using the MOCVD source gases. This barrier layer blocks the diffusion of hydrogen into the material. The reactor is then cooled in a reactive or non-reactive ambient gas. The semiconductor material is then removed from the reactor with little or no passivation of the dopant species. The barrier layer may comprise a silicon nitride layer. The Proceedings from the MRS Fall 2004 Meeting, Symposium E (GaN, AlN, InN and Their Alloys), Vol. 831 (Vol. 831 Åbstracts due by Jun. 22, 2004, Meeting held November 29-Dec. 3, 2004), Abstract E6.7 and E8.20 discuss AlGaN/GaN HEMTs that include an SiO2 or Si3N4 passivation by both PECVD and by in-situ MOCVD of the top AlGaN surface as a way of improving device performance, and reported that 2DEG carrier concentration increased strongly with increasing Si3N4 thickness, varying from 0 to 15 nm.